Conventional RF and microwave narrow-band bandstop filters generally consist of a length of transmission line or waveguide to which multiple one-port bandstop resonators are coupled--either by direct contact, by probe, by loop, or by iris--at spacings of approximately an odd multiple of a quarter wavelength, usually either one quarter wavelength or three quarter wavelengths. The individual resonators are typically quarter-wavelength transmission line resonators or dielectric resonators.
It is also known to provide some means of tuning the frequency of the resonators, since manufacturing tolerances and material properties make resonator frequencies too unpredictable to guarantee optimum filter performance. Usually, the characteristic impedance of the transmission line is held constant along its length. Filters have been implemented utilizing stripline technology resulting from a design method which produces very specific impedance values in a stepped impedance transmission line. (Schiffman and Young, "Design Tables for an Ecliptic Function Band - Stop Filter", IEEE Vol. MTT-14 No. 101,966 page 474). Such designs, however, tend to suffer from a more complex configuration, stingent dimensional tolerances, unsuitability to narrow band applications and excessive panband losses.
With prior art narrow-band bandstop filters, the unloaded Q of all of the resonators must be maximized to achieve the best performance, while their level of coupling to the transmission line must be individually adjusted to obtain the best performance. Unfortunately, given a transmission line of constant impedance, the optimum values of these couplings may exceed the maximum achievable, or desirable, with a given coupling method. For a fixed number of resonators, the performance of the filter then becomes limited by the maximum achievable coupling rather than by maximum obtainable unload Q of the resonators. Under such circumstances, the optimum filter performance cannot be realized.
While equal-ripple stopband, constant-impedance transmission line notch filters are known, and given a maximum achievable or desirable level of coupling of the resonators to the transmission line, it would be desirable to achieve:
similar or better performance (notch depth, selectivity, and bandwidth) with fewer resonators, PA1 greater notch selectivity (ratio of notch floor width to width between passband edges) with similar or better notch depth, PA1 and greater notch depth (greater level of band rejection) with similar or better notch selectivity.
In addition, from a manufacturing and installation point of view, it is desirable to achieve reduced sensitivity of each resonator's characteristic resonant frequency to the coupling mechanism which couples between the resonator and the transmission line. This would provide improved mechanical and temperature stability for the filters, better repeatability of electrical performance from device to device, and less interaction between the tuning of the coupling and the tuning of the resonant frequency of a resonator.
While constant impedance transmission line notch filters are known, it would be desirable to be able to achieve similar levels of performance but with fewer resonators. Further, it would be desirable to achieve greater notch depth, that is greater level of band rejection, with the same number of resonators as utilized in constant impedance transmission line notch filters with similar bandwidth and bandedge attenuation.
Further, it would be desirable to be able to create a variety of notch filters using a plurality of relatively standard elements such as resonators, transmission line segments and the like without having to create a large variety of specialized components which are only usable with a given filter design.